The present invention relates generally to semiconductor processing and, more particularly, to improved techniques for fabricating gate dielectrics.
As integrated circuits have become smaller and more densely packed, so have the dielectric layers of devices such as field effect transistors and capacitors. With the arrival of ULSI (Ultra Large Scale Integrated circuit) technology and gate dielectrics of less than 15 angstroms (xc3x85) in thickness, the use of silicon dioxide (SiO2) as a traditional gate dielectric material becomes problematic.
In larger devices (e.g., where the gate oxide thickness is 40 xc3x85 or more), leakage currents from a polysilicon gate electrode, through the gate oxide and into the device channel, are only on the order of about 1xc3x9710xe2x88x9212 A/cm2. However, as the thickness of an SiO2 gate dielectric is decreased below 20 xc3x85, the leakage currents approach values of about 1 A/cm2. This magnitude of leakage current, caused by direct tunneling of electrons from the polysilicon gate electrode through the gate oxide, results in prohibitive power consumption of the transistor(s) in the off-state, as well as device reliability concerns over an extended period of time.
Another problem with ultrathin SiO2 gate dielectrics relates to the doping of the polysilicon gate electrodes with a dopant material, such as boron. Such doping is typically used to combat channel depletion effects which cause voltage threshold (Vt) shifts and higher threshold voltages. With an ultrathin SiO2 gate dielectric, however, the boron dopant atoms can easily penetrate the SiO2 layer and thereby cause large Vt shifts and reliability problems themselves.
Accordingly, the nitrogen doping of gate dielectrics has become a preferred technique of semiconductor chip manufacturers. For gate dielectrics having a thickness range of about 15 xc3x85 to 20 xc3x85, silicon oxynitride (SiOxNy) layers have replaced SiO2 layers as the choice of gate dielectric material. The beneficial effects of nitrogen incorporation into the dielectric are generally dependent upon the concentration of the doping and the distribution of the doping profile relative to both the Si/SiO2 interface and the polysilicon gate/SiO2 interface. If properly carried out, the nitrogen doping reduces leakage current and boron penetration, while minimizing or negating the impact on Vt and channel electron mobility.
Present nitridation techniques, however, do have certain drawbacks associated therewith. For example, a rapid thermal annealing process (such as in the presence of N2O or NO gas) by itself may not result in a sufficiently high nitrogen content so as to faciliate the desired reduction in leakage current. In the case of a plasma process, such as remote plasma nitridation (RPN), the possibility exists that the ionized plasma species will cause damage to active devices formed on the semiconductor wafer.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for forming an ultra thin gate dielectric for an integrated circuit device. In an exemplary embodiment of the invention, the method includes forming an initial nitride layer upon a substrate and then re-oxidizing the initial nitride layer, thereby forming an oxynitride layer. The oxynitride layer has a nitrogen concentration therein of at least 1.0xc3x971015 atoms/cm2 and has a thickness which may be controlled within a sub 10 xc3x85 range.
In a preferred embodiment, forming the initial nitride layer includes rapidly heating the substrate in the presence of an ammonia (NH3) gas at temperature of about 650xc2x0 C. to about 1000xc2x0 C., and at a pressure of about 1 Torr to about 760 Torr. Re-oxidizing the initial nitride layer includes rapidly heating the initial nitride layer in the presence of a nitric oxide (NO) gas at temperature of about 650xc2x0 C. to about 1000xc2x0 C., and at a pressure of about 1 Torr to about 760 Torr. The oxynitride layer preferably has a nitrogen atom concentration of about 1.0xc3x971015 atoms/cm2 to about 6.0xc3x971015 atoms/cm2.